Controllable high speed journal bearings, lubricated with electro-rheological fluids. An analytical and experimental approach

The problem of journal bearings control is of great importance in mechanical engineering. A very recent method for doing this is the creation of ‘smart' journal bearings using electro-rheological (ER) fluids. If such a fluid is used to lubricate a journal bearing system, it is expected that the imposition of an electric field between the rotor and the stator will cause an alteration in the dynamic properties of the journal bearing. In this paper an experiment in a high speed journal bearing (16 000 to 35 000 s⁻¹), with small radial clearance is presented. The alternation of the attitude locus (eccentricity and attitude angle) and the stiffness coefficients in a loaded journal bearing lubricated with ER fluid is investigated and presented. The Reynolds equation is solved using the finite element method in order to get the dynamic characteristics of the ER bearings vs the electric field and to simulate its dynamic behavior. The Bingham plastic model of non-Newtonian fluid flow behavior is used to described the ER lubricant. The accuracy of the algorithm is obtained by comparing the results published by previous investigators and the experimental data described in this paper. It is concluded that ER fluids can be used to create ‘smart' journal bearings. and vibration controllers can be constructed to control the dynamics and stability of the ER fluid lubricated bearings.     

A Simulation Study on the Behavior of Magnetorheological Fluid on Herringbone-Grooved Hybrid Slot-Entry Bearing

Abstract

In recent years, extensive use of smart lubricants has been made in order to control the tribological performance of fluid film bearings. The grooved surfaces of the journal bearing greatly influence the performance of bearings. In the present work, various geometric shapes of herringbone grooves (rectangular, triangular, and parabolic) with groove angles (30° and 60°) have been considered to numerically simulate the performance of slot-entry bearings. The work reported in this article deals with the numerical simulation of magnetorheological (MR) fluid–lubricated slot-entry herringbone-grooved hybrid journal bearings. Dave equation, a constitutive relation of the Bingham model, was employed to simulate the flow behavior of MR fluid. Using the finite element method (FEM), the governing Reynolds equation for a hybrid slot-entry bearing model was solved. The result shows that the use of a herringbone-grooved surface and application of MR fluid in a slot-entry bearing offers better stability and higher fluid film stiffness and minimizes frictional torque.     

Effects of electric field on hydrodynamic characteristics of finite-length ER journal bearings

A numerical analysis is performed on the hydrodynamic characteristics of electrorheological (ER) fluid flows in journal bearings based on computational fluid dynamics (CFD) techniques. The bearing has a finite-length and operates under incompressible laminar flow with steady conditions. The analysis is based on the numerical solution of full three-dimensional Navier–Stokes equations. The applied electric field is imposed partially along a contractive section of the journal bearings and the modified Bingham plastic model is used to describe the behavior of ER fluid. The results presented are obtained by using finite-volume-methods and solved by the semi-implicit method for pressure-linked equations (SIMPLE) algorithm. In this study, cavitation effects are also considered by using the viscosity modeling. The results have shown that the influences of ER effects, caused by the applied electric field, on the journal bearing characteristics are significant and not negligible. Compared with the Newtonian lubricant case, the effects of the ER effects provide an enhancement in the load-carrying capacity, but do have little effects on the attitude angle. The quantitative effects on load-carrying capacity are more pronounced for journal bearings operating at higher values of eccentricity ratio or lower rotary speed.     

The Static and Dynamic Characteristics of a Two-Lobe Journal Bearing Lubricated with Couple-Stress Fluid

Static and dynamic characteristics of two-lobe journal bearings lubricated with couple-stress fluids are studied. The load-carrying capacity, the stiffness and damping coefficients, the non-dimensional critical mass, and the whirl ratio are determined for various values of the couple stress parameter l. The results obtained are compared with the characteristics of two-lobe bearings lubricated with Newtonian fluids. It is found that the effect of the couple stress parameter is very significant on the performance of the journal bearing. The stability is improved compared to bearings lubricated with Newtonian fluids.     

Linear stability analysis of hydrodynamic journal bearings under micropolar lubrication

The dynamic characteristics of hydrodynamic journal bearings lubricated with micropolar fluids are presented. The modified Reynolds equation is obtained using the micropolar lubrication theory. Applying the first order perturbation of the film thickness and steady state film pressure, the dynamic characteristics in terms of the components of stiffness and damping coefficients, critical mass parameter and whirl ratio are obtained with respect to the micropolar property for varying eccentricity ratios and slenderness ratios. The results show that micropolar fluid exhibits better stability in comparison with Newtonian fluid.     

Effects of magnetic fields on the dynamic characteristics of wide parallel step slider bearings with electrically conducting fluids

By the use of electrically conducting fluids as lubricants together with the application of externally magnetic fields, the dynamic lubrication problems of parallel step slider bearings lubricated with a nonconducting fluid are extended in this paper. A closed‐form solution has been derived for the magneto‐hydrodynamic characteristics of wide parallel step slider bearings. Comparing with the bearing lubricated with nonconducting fluids, the magneto‐hydrodynamic parallel step slider bearings signify an improvement in the load capacity, as well as the dynamic stiffness and damping coefficients. A numerical example and calculated values are also provided in tables for engineers in bearing selection and designing. Copyright © 2012 John Wiley & Sons, Ltd.     

Linear Stability Analysis of a Rough Short Journal Bearing Lubricated with Non-Newtonian Fluids

On the basis of Stokes theory and the Christensen's stochastic model, the combined effects of couple stresses and surface roughness on the instability threshold of hydrodynamic rotor bearings are theoretically studied. The film pressure is solved from the generalized stochastic non-Newtonian Reynolds-type equation and applied to evaluate the equilibrium solution of a short journal bearing. By using the linear theory, the eight oil-film stiffness and damping coefficients as well as the vibrational stability boundary were determined. According to the results obtained, the presence of couple stresses accompanied with a longitudinal roughness provides an increase in the values of stiffness coefficientsK _XX , K _XY and K _YX as well as the damping coefficient C _XX , and results in a higher stability threshold speed. Whereas, when compared to the smooth bearing lubricated with couple stress fluid, a decrease in the threshold speed is found in the case of transverse roughness.     

Misalignment Effect on the Static Characteristics of Magnetized Journal Bearing Lubricated with Ferrofluid

In this paper a generalized Reynolds equation suitable for misaligned hydrodynamic journal bearings lubricated by either Newtonian or non-Newtonian ferrofluid is developed. The equation has been solved numerically by the finite difference technique with an appropriate iterative method and the pressure distributions have been obtained. Design charts for all static characteristics (including load capacity, attitude angle, side flow, friction force and misaligned moments) under different misalignment conditions are presented. The effects of misalignment conditions on all these bearing-performance characteristics are analyzed. The results conclude that magnetic lubrication improves, in general, the bearing performance under misalignment.     

Dynamic Friction Model of Lubricated Surfaces for Precise Motion Control

A model is developed to describe dynamic friction effects in lubricated surfaces. The model covers the hydrodynamic, mixed and boundary lubrication regions. The dynamic friction model can predict the friction force for time-varying velocity, and is useful in precise motion control. The model presented is for a short journal bearing, but can be extended to other geometries of sliding surfaces, such as point and line contacts or rolling element bearings. The friction is related to a time variable fluid film thickness, resulting from journal vibrations relative to the sleeve. The proposed model agrees qualitatively with experimental results for lubricated line contact. Both show similar hysteresis-type friction curves under oscillating velocity.     

表面织构动压滑动轴承油膜力解析模型

提出一种求解表面织构动压轴承油膜力的解析模型。基于Sommerfeld油膜边界,通过分离变量的方法,求解表面织构动压滑动轴承二阶偏微分Reynolds方程,得到表面织构动压滑动轴承油膜压力解析式。以圆形凹坑轴承为例,在油膜区域通过积分求得织构轴承的油膜力,分析织构参数对油膜压力的影响,研究发现,表面织构位于收敛区域(升压区)的轴承,其润滑与承载性能优于表面织构位于发散区域(降压区)的轴承、全织构轴承以及光滑轴承。对比了提出的解析模型与FDM和CFD模型在不同长径比和偏心率下的计算结果,结果表明,提出的解析模型能准确地描述表面织构动压滑动轴承的油膜力,且计算结果同FDM和CFD模型计算结果基本一致,验证了该模型的正确性。     

A study of misaligned electrorheological fluid lubricated hole-entry hybrid journal bearing

The present work describes a theoretical study to demonstrate the effect of the journal misalignment on the performance of an orifice compensated hole-entry hybrid journal bearing system lubricated with ER fluid. The journal misalignment has been accounted for by defining a pair of misalignment parameters sigma and delta. Continuous Bingham Model has been used to describe the behavior of ER fluid. The bearing static and dynamic characteristic parameters presented in the study indicate that the effect of journal misalignment is in general to cause a reduction in bearing dynamic characteristic parameters whereas the electric field in the ER fluid is to partially compensate this loss.     

Linear Stability Analysis of Hydrodynamic Journal Bearings with a Flexible Liner and Micropolar Lubrication

A linear stability analysis of hydrodynamic journal bearings is presented, including the effects of elastic distortion of the liner and micropolar lubrication. Hydrodynamic equations of the lubricant and equations of motion of the journal are solved simultaneously with the deformation equations for the bearing surface to predict the fluid film pressure distributions theoretically. The components of stiffness and damping coefficients, critical mass parameter, and whirl ratio, which reflect the dynamic characteristic of the journal bearing, are calculated for varying eccentricity ratio taking into account the flexibility of the liner and the micropolar properties of the lubricant. The results presented show that stability decreases with an increase in the value of the elasticity parameter of the bearing liner and micropolar fluids exhibit better stability in comparison to Newtonian fluids.     

Feasibility Study of Hydrodynamic Lubrication in Supporting of Rotating Elements of Galvanizing Hardware Submerged in Molten Zinc

Severe wear of sink roll supports in continuous galvanizing lines has been a cause for concern for some years. This wear, which affects the quality of the coating and downtime costs, is a result of a corrosive environment and highly loaded surface contact.

This paper presents results from a feasibility study to use journal bearings lubricated with molten zinc to separate the interacting surfaces. Analytical and experimental investigations were aimed at identifying the load capacity of sink roll bearings when operating in the hydrodynamic regime, as well as the effect of the molten zinc bath environment on bearing performance. This environment includes chemical zinc attack and the presence of inter-metallic particles in the bath.

The study indicates that it is difficult to achieve hydrodynamic bearing operation for the entire range of operating conditions. It also shows that, when selecting materials for journal bearings support to the rotating elements submerged in molten zinc baths, possible contact of the bearing surfaces as well as chemical inertness and resistance to the hard intermetallic particles do need to be addressed.     

Computation of dynamic force coefficients for hybrid (hydrostatic/hydrodynamic) journal bearings by the finite disturbance and perturbation techniques

This paper presents non-dimensional linearized bearing coefficients for oil lubricated plain line-entry (slot) hybrid journal bearings. This represents the first stage of a programme of work on the dynamic characteristics of plain line-entry hybrid journal bearings. A full-scale dynamic analysis of this type of bearing, both theoretical and experimental is currently underway. Two techniques for predicting the linearized bearing coefficients are presented and compared: (i) the finite disturbance technique; (ii) the perturbation technique. Both analyses are based on a finite-difference approximation of the classical Reynolds equation, with provision for source flow and fluid-film rupture, although an isoviscous fluid is assumed.     

CFD analysis of the performance of elbow-meter with high concentration coal ash slurries

Elbow meter is a simple flow measuring device and its characteristics for the flow of single-phase fluids are reasonably well understood and the functional dependence of elbow meter coefficient (C_k) on parameters like Reynolds Number, radius ratio, pipe roughness etc. Is well documented in literature. Elbow meters are also being used for solid liquid flow in many industries. The present study aims to establish the characteristics of an elbow meter for high concentration coal ash slurry pipelines using validated CFD. High concentration coal ash slurries are known to behave as homogeneous fluids exhibiting behavior as Bingham plastic fluids. The validated CFD methodology has been used to predict the values of C_k for the flow of Bingham plastic fluid and establish its dependence on radius ratio, Hedstrom Number and Bingham Reynolds Number. Further, for the flow of high concentration fly ash slurry flows, C_k for any given radius ratio is observed to be independent of Hedstrom Number (over the range investigated He ≤ 10⁵). Further, in fully turbulent flows, beyond a critical Reynolds number (Re_B ≥ 5.3 × 10³), C_k remains constant and is dependent only on the radius ratio.     

A Study of Non-Recessed Hybrid Flexible Journal Bearing With Different Restrictors

For bearings operating under heavy loads, the elastic deformation of bearing surface induced by fluid film pressures can no longer be neglected as it is comparable to the order of magnitude of fluid film thickness. In the present work a theoretical study describing comparative performance of non-recessed hybrid journal bearing using different flow control devices has been carried out by considering bearing shell flexibility into the analysis. The relevant governing equations have been solved using finite element method. The comparative performance of non-recessed hybrid journal bearings of two separate configurations have been studied for various values of bearing flexibility parameter (([Cbar]_d)). The results have been presented for hole-entry type journal bearings compensated by capillary, orifice and constant flow valve restrictors and for a slot-entry type journal bearing, for the same set of values of operating and geometric parameters. The computed results indicates that in order to get an improved performance of non-recessed journal bearing, a proper selection of bearing flexibility parameter (([Cbar]_d)) along with type of flow control device (i.e., capillary, orifice, constant flow valve, slot etc.) and type of bearing configuration (symmetric/asymmetric) are essential.     

Application of computational fluid dynamics and fluid structure interaction techniques for calculating the 3D transient flow of journal bearings coupled with rotor systems

Journal bearings are important parts to keep the high dynamic performance of rotor machinery. Some methods have already been proposed to analysis the flow field of journal bearings, and in most of these methods simplified physical model and classic Reynolds equation are always applied. While the application of the general computational fluid dynamics (CFD)-fluid structure interaction (FSI) techniques is more beneficial for analysis of the fluid field in a journal bearing when more detailed solutions are needed. This paper deals with the quasi-coupling calculation of transient fluid dynamics of oil film in journal bearings and rotor dynamics with CFD-FSI techniques. The fluid dynamics of oil film is calculated by applying the so-called ’’dynamic mesh’’ technique. A new mesh movement approach is presented while the dynamic mesh models provided by FLUENT are not suitable for the transient oil flow in journal bearings. The proposed mesh movement approach is based on the structured mesh. When the journal moves, the movement distance of every grid in the flow field of bearing can be calculated, and then the update of the volume mesh can be handled automatically by user defined function (UDF). The journal displacement at each time step is obtained by solving the moving equations of the rotor-bearing system under the known oil film force condition. A case study is carried out to calculate the locus of the journal center and pressure distribution of the journal in order to prove the feasibility of this method. The calculating results indicate that the proposed method can predict the transient flow field of a journal bearing in a rotor-bearing system where more realistic models are involved. The presented calculation method provides a basis for studying the nonlinear dynamic behavior of a general rotor-bearing system.     

考虑粗糙度的船舶水润滑高分子轴承弹流润滑性能研究

为了揭示表面粗糙度对船舶水润滑高分子材料轴承润滑性能的影响规律,开展水润滑轴承弹流混合润滑理论研究;建立考虑内衬材料粗糙度和弹性变形的水润滑轴承混合润滑模型,并对模型进行仿真验证;分析内衬粗糙峰对水膜厚度、水膜压力分布和承载能力的影响规律。研究结果表明：在转速增大的过程中,内衬粗糙度的增大会减缓水膜厚度的增幅比,使轴承需要更高的转速来进入流体动压润滑状态;减小轴承内衬粗糙度能有效降低轴承起飞转速,加快轴承由混合润滑转变为流体动压润滑的过程,减小轴承与轴颈的局部接触,降低轴承异常振动噪声发生的可能性。研究结果揭示了内衬粗糙度变化对轴承润滑特性的影响机制,为水润滑轴承的优化设计提供理论参考。     

Non-linear dynamics of a rigid,unloaded andunbalanced rotor on lubricated bearings

The dynamic behaviour and stability analysis of an unloaded rotoron cylindrical lubricated bearings subjected only to the actionof unbalance is presented. The ``short bearing model' has beenadopted to evaluate the hydrodynamic fluid film force. As theunbalance increases, the system shows the typical behaviour ofnon-linear mechanical systems as the non-linearity of the hydrodynamic force field becomes more important. Evaluating theapproximate circular and centred analytical expressions of thetrajectory, the frequency response curves of the amplitude ofthe synchronous orbit have been reported. The stability limitcurve of the synchronous response has been obtained by means ofthe Floquet theory. Several examples of journal trajectories,obtained through step-by-step integration of the motionequations, are reported to illustrate the dynamic behaviour ofthe system and to verify the validity of the stability limitcurve.     

Influence of lubricants on the performance of journal bearings – a review

ABSTRACT

The lubricant properties have a significant influence on the static and dynamic performance characteristics of journal bearing such as load-carrying capability, minimum fluid film thickness, maximum pressure, lubricant flow rate, damping coefficients, stiffness coefficients, etc. The present document reviews the behaviour of various lubricants such as power-law lubricants, couple stress lubricants, micropolar lubricants, ionic liquid lubricants and space lubricants. The influence of these lubricants on the performance of hydrostatic, hydrodynamic and hybrid journal bearings is discussed. An effort is made to develop the understanding to choose the suitable lubricant for journal bearings for different journal bearing configurations. Journal bearings operated with non-Newtonian lubricants have shown better performance compared to Newtonian lubricants. Ionic liquid lubricants have shown high potential in vacuum applications and extreme temperature environment such as in bearings of spacecraft moving mechanical assemblies.